The present invention relates to a convolutional encoder, particularly to such an encoder having a high code rate efficiency.
Convolutional code rate efficiency can be improved without decreasing their minimum Hamming distance. This is achieved by increasing the constraint length of the convolutional codes. IEEE Transactions on Information Theory, Vol. 23 No. 5, 1977, pages 615 to 618 describes a convolutional encoding technique for such purposes with a minimum Hamming distance of 4, a code rate 7/8 and a constraint length 56.
Punctured convolutional encoding is known as a high code rate efficiency technique. With this technique, a 7/8 convolutional encoder having a minimum Hamming distance of 3 and 64 states is described in Sixth International Conference on Digitlal Satellite Communications, Session 12, pages 24 to 31.
However, the number of sates that a convolutional encocer assumes increases exponentially as a function of the constraint length ofthe encoder and for this reason it is impractical for decoding large constraint length codes using Viterbi decoding algorithm which is known as a maximum likelihood decoding method. Convolutional codes having a large constraint length are undesirable since relatively large delays are introduced to decoding process.